A standard chuck for a large workpiece has a chuck body rotatable about and centered on an axis and a plurality of angularly spaced jaws radially displaceable on the chuck body. Means is provided for synchronously moving the jaws inward or outward so as to inwardly or outwardly engage a surface of the workpiece and retain it on the chuck body which is then rotated while a tool is engaged with the workpiece to cut, grind, finish, or otherwise machine it.
When turning a large rotation-symmetrical workpiece like a brake disk, flywheel, ring gear, bearing ring, or the like it is necessary to rotate the workpiece about its symmetry axis. At the same time the workpiece must be held solidly enough that, when it is engaged by a cutting, grinding, or other tool it will move in the chuck. Finally it is critical that the force brought to bear to hold the workpiece not be so great as to deform it.
The existing chucks have the considerable disadvantage that, the workpiece has to be very carefully centered on the chuck body before the jaws are brought into engagement with it. If it is not perfectly positioned it is possible for some of the jaws to engage it and hold it before others so that the workpiece will be deformed. If machined while deformed, the workpiece will not be rotation symmetrical when it returns to its normal shape after being dechucked.